As a hard-won fan of the series, this review is very difficult for me to write, as I didn't enjoy this book as much as the others.
But as I am an aspiring writer myself, I've become a huge supporter of frank, constructive criticism. That said, I'll try to be as succinct as possible - and not too much of an insensitive jerk.
I had two major problems with Doubletake.
The first problem isn't with the bond between Cal and Niko, or their relationships with Robin or Promise - that remains pretty much the same. The problem is the story. The plot takes a lot of interesting turns in this book, and unfortunately a lot of them are fairly pointless. For instance, in one chapter, Robin hires Cal and Niko to work as bouncers while his entire race comes together to decide which of them will reproduce. You would think that a great gathering of the Pucks, the oldest and most devious race of tricksters on the planet, would play an important role in the story. Not so much. It pretty much just turns out to be a giant, disgusting orgy that goes on way too long and described with way too much detail. And after the important gathering/orgy is over with, that's it. Nothing. There's no intrigue - no pucks plotting against one another. No thousand-year-old grudges get settled. Nothing. We got a few tidbits about Robin's past, but nothing else.
And that's just one example. I would expect to encounter things like that in well-written (but ultimately misguided) fan fiction, not from the real author!
The second problem was the villain. Of course, as per usual, there is more than one bad guy for the brother's to tackle, but there is always a Big Bad. In this book, it's a man called Grimm. He's pretty much exactly like Caliban but in the all the worse ways. Kind of like Professor Moriarty - all of Sherlock Holmes' abilities, with none of his morality.
The very idea of Grimm makes my spine tingle with anticipation. Finally! Someone that can really challenge Caliban! None of that crap where Cal just makes a gate and boom - bad guy dead. Unfortunately, Grimm is pretty harmless. We get to hear about all the people's he's killed and eaten and whatnot, but we don't really get to see it. I'm sure all of the horrid experiences he describes are true, but talk is just that: talk. I want action. Show me that he's a bad mofo - don't just tell me. Add that to the fact that because his master plan for world domination relies partly on Caliban's participation, thereby keeping him from causing any serious harm to Cal or his friends....like I said, harmless. Virtually every scene they share together is him making big threats and then being unable to follow up on them while Cal kicks his butt at every opportunity. You can't bring Moriarty into the story and then tell him he has to follow certain rules.
That's pretty much where my complaints end. There are some other annoyances, one of them being TOLD about a semi-important character's demise at the hands of Grimm. And Delilah's absence.
All in all, I'd say I am not mad that I read it, but I'm certainly not satisfied either. There was a lot I could have done without *cough* orgy *cough*, and there was even more I really wanted to see but didn't. If you're a fan of the series, there are a few important plot developments you'll want to be aware of in Doubletake, and plus it's always fun to listen to Cal, Niko, and Robin joke and banter with each other while facing certain death. And also, we get an extremely small hint of another romantic candidate for Cal - God knows he needs one.
If you're new to the series, don't start with this book. Ideally, you would start with Nightlife, the first book in the Cal Leandros series, but if you're on the fence about it, I'd recommend starting with Blackout first. It's the strongest book in the series to date. You know...in my opinion.